In order to safely administer anesthetic gases to a patient under anesthesia, anesthesiologists need to be able to monitor the concentration of certain gases which patients inhale and exhale to prevent the risk of supplying too much or too little gas. Carbon dioxide (CO.sub.2) is not administered, but it and the administered anesthetic gases nitrous oxide (N.sub.2 O), halothane, enflurane, isoflurane, sevoflurane, and desflurane are monitored. These anesthetic gases may be administered as a single agent gas, or as a mixture of agent gases.
Because expired CO.sub.2 is a reliable indicator of the carbon dioxide concentration in the arterial blood, the concentration of expired CO.sub.2 is often the most critical of the gases to observe. This supervision helps to prevent excess CO.sub.2 from being delivered to the patient, by preventing malfunctions in the anesthetic breathing apparatus.
With the help of modern technology, gas analyzers which utilize infrared radiation have been developed to measure the concentration of CO.sub.2 and the anesthetic gases. These gas analyzers take advantage of the known infrared absorption characteristics of CO.sub.2 and the anesthetic gases to determine which gases are present. In other words, in a typical gas analyzer, an infrared light would be emitted through a respiratory gas stream in the main (or side) airway, and onto an infrared sensor/detector device which contain certain infrared bandpass filters and a set of thermopiles to ultimately determine the concentration of carbon dioxide or any of the anesthetic gases. For convenience purposes, each combination of an infrared bandpass filter and its respective thermopile will be referred to as a "channel". In this respect, a "channel" thus does not define an actual physical pathway, but rather, is an imaginary pathway arbitrarily defined as in the previous sentence in order to easier describe the invention herein.
One example of such analyzers includes that disclosed in U.S. Pat. No. 5,081,998 entitled OPTICALLY STABILIZED INFRARED ENERGY DETECTOR (the disclosure of which is incorporated herein by reference). As shown in FIG. 1, this analyzer uses pairs-of thermopiles connected in series opposed relation, with the first and second thermopiles of each pair located next to each other, whereby each thermopile is preceded by an optical bandpass or a neutral density filter to permit different infrared radiation wavelengths to reach each thermopile in the pair. This resulting difference in output is then used to eliminate the effects of background thermal noise. However, difficulties were encountered due to the space limitations in trying to arrange six or more independent analytical channels to detect additional anesthetic gases over a restricted mounting area.
This space arrangement problem was apparently improved upon in the apparatus disclosed in U.S. Pat. No. 5,296,706, entitled SHUTTERLESS MAINSTREAM DISCRIMINATING ANESTHETIC AGENT ANALYZER (the disclosure of which is incorporated herein by reference). As shown in FIG. 2, this apparatus uses a first and second thermopile connected in a "parallel opposed" fashion for each independent detector channel. In other words, there are two thermopiles for every detector channel: the first thermopile located in the path of the incident light and the second thermopile shielded from all incident light and located directly behind the first. The purpose of the second thermopile is to produce reference output representative of ambient temperature transients. Upon detection of carbon dioxide and the anesthetic gases, the concentrations of the gases are then calculated using a second order polynomial equation having cross product terms.
The present inventors have found that it was difficult to construct a sensor with seven to ten channels, each with a reference detector connected in a parallel opposed manner directly behind its corresponding infrared detector. The present inventors also found that the use of large order polynomials to calibrate and compensate the measurement was difficult to accomplish.